Mass spectrometry



Nov. 9, 1954 c. E. BERRY MASS SPECTROMETRY FiledJune 26, 1953 8 E 3 m2 5 T B M B M M R M 4 U 2 u E TO VACUUM FRE QUE NC E NE RA TOR C/LLATOR OUTPUT SIGNAL 53 INVENTOR.

CLIFFORD E. BERRY A v, C M M uR 00 7 LM mFm6 R H mm m HHG A2 m6 A m R E 2 M xua 6 6 E6 0 PF A T TOPNE Y United States Patent MASS SPECTROMETRY Clifford E. Berry, Altadena, Califi, assignor to Consoli-.

dated Engineering Corporation, Pasadena, Calif., a corporation of California Application June 26, 1953, Serial No. 364,253

8 Claims. (Cl. 25041.9)

under investigation, spatial temporal or directional separation of such ions as a function of mass-to-charge ratio, and selective collection of ions of one or more given specific masses. In the mass spectrum analysis of gas samples, sample molecules have generally been ionized by means of an electron beam. In dealing with solid samples, the problem of presenting molecules of the sample for ionization is involved. It is, of course, essential that the ionized portion of the sample be truly representative of the sample composition. Selective vaporization or ionization of the sample constituents must be avoided.

It has heretofore been proposed to produce neutral molecules from a solid sample by means of a spark generated between two electrodes, at least one of which electrodes constitutes the sample. The molecules thus developed are then caused to flow into an ionization region for ionization by conventional means such as electron bombardment.

One of the disadvantages of this system is that gases evolved from the walls of the chamber in which the electrodes are mounted as a consequence of heating or ion bombardment also pass into the influence of the ionizing medium. In this fashion a spurious background unrelated to the composition of the solid sample is produced to which a conventional collection system is not selective.

I have devised a comparatively simple method for avoiding the error inherent in a variable spurious background of this nature. In accordance with the invention, I provide a mass spectrometer for the analysis of solid samples which comprises an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, and a collector electrode disposed in the analyzer chamber. The ionization chamber is provided with means for propelling ions therefrom into and through the analyzer chamber whereby ions of a given mass-tocharge ratio may be caused to strike and discharge on the collector electrode. A molecule generating chamber is associated with the ionization chamber and opens therein. A pair of electrodes are mounted in the molecule generating chamber and at least one of these electrodes is composed of the solid sample to be analyzed. The electrodes are associated with means for periodically developing a spark therebetween and a regulating means for determining the repetition rate of the spark. An alternating current amplifying means is connected to the collector electrode for amplifying the alternating components of the discharge current and including means for limiting the amplified signal to a frequency corresponding to the repetition rate of the spark.

In the apparatus of the invention, the collection system, including amplification means, is rendered insensitive to ionization produced other than at the intermittent intervals of molecule generation at the spark. The required synchronization between the collection means and the spark generating means may be accomplished with a tuned amplifier or by means of a synchronous detector connected to the output of the amplifier and to the spark generating circuit. In this manner the collection system, which term is used as encompassing collector electrode and amplifying means, is insensitive to the D. C. or low frequency background disturbance.

The invention will be more fully understood from the 2 7 following detailed description thereof, taken in conjunction with the accompanying drawing, in which:

Fig. l is a diagrammatic view of apparatus of the present invention showing certain components of a mass spectrometer tube in section;

Fig. 2 is a section taken on the line 22 of Fig.1; and

Fig. 3 is a partial diagrammatic view representing a modification of the apparatus of Fig. 1.

Referring to Fig. l, the mass spectrometer in accordance with one embodiment of the invention comprises an ionization chamber 10, a serially arranged analyzer chamber 11 having a collector electrode 12 disposed in the end thereof remote from the ionization chamber. A resolving electrode 13 is disposed in the analyzer chamber in front of the collector electrode to resolve spatially separated ion beams passing through the analyzer chamber from the ionization chamber. The ionization chamber is provided with a repeller electrode 14 and accelerating electrodes 15, 16. An electron gun 17 (Fig. 2) is mounted adjacent one side of the ionization chamber and directs an electron beam 18 across the ionization chamber through apertures 10A, 10B in opposite walls thereof. The electron beam traverses the ionization chamber between repeller electrode 14 and the first accelerating electrode 15. A target electrode 20 is disposed adjacent aperture 10B in the ionization chamber for discharge of electrons passing through the chamber.

Repeller electrode 14 and accelerating electrodes 15, 16 are connected through suitable leads to a voltage supply circuit 22 by means of which proper potentials are impressed on these electrodes to discharge ions from the ionization chamber at a high velocity into the analyzer tube. Voltage supply circuit 22 comprises a voltage source illustrated as battery 23 connected across a slide wire 24 to which the several electrodes in the ionization chamber are connected by suitable taps.

A molecule generating chamber 26 is disposed adjacent ionization chamber 10 and opens into the ionization chamber through an aperture 10C in a wall thereof, aperture 10C being in alignment with apertures 10A, 108 so that molecules entering the ionization chamber from the molecule generating chamber pass into the influence of electron beam 18. A pair of electrodes 27, 28 are mounted in the molecule generating chamber 26, at least one of these electrodes comprising the sample to be analyzed. These two electrodes are connected by suitable leads to a voltage generating circuit.

The ionization chamber, analyzer tube and molecule generating chamber are all enclosed within an envelope 30 which is connected through a suitable conduit 31 to evacuating means (not shown) whereby the entire system may be maintained .under a high degree of vacuum. The analyzer chamber of the instrument is immersed in a transverse magnetic field established by opposite magnet poles, one of which is illustrated in Fig. 1 and identified by the reference character 32.

In the embodiment illustrated in Fig. l, a timing os-' cillator 34 is connected to deliver an alternating signal illustrated by the wave form 34A to a pulse former 36 and to a synchronous detector 38. Pulse former 36 converts the alternating wave 34A to a pulsed wave illustrated at 36A, and feeds'this wave to a high voltage, high frequency generator 40 as a timing or trigger signal. This generator develops a continuous high voltage, high frequency signal which is triggered by the impulse from the pulse former to produce a wave form illustrated at 40A which is supplied to electrodes 27, 28 to periodically develop a spark between the electrodes.

Collector electrode 12 is connected to an alternating current amplifier 42, the output of which is fed to synchronous detector 38. The output of the synchronous detector compirses a D. C. signal representing only the amplified A. C. signals received at the collector electrode and of a frequency corresponding to the repetition rate of the molecule generating spark as in turn determined by the frequency of the oscillator 34. In a typical operation, the oscillator 34 may be set at a frequency of approximately 30 C. P. S. and the high frequency generator may develop a signal of a frequency of approximately 1 megacycle. limiting to the invention.

Patented Nov. 9, 1954 These values are typical but are in no Way In operation the instrument of Fig. 1 functions as follows? A high voltage signal of the configuration lllustrated at 40A is impressed on the spark generating electrodes to develop an intermittent spark of a periodicity corresponding to the frequency of the tr1ggering signal developed by the oscillator. The spark produces neutral molecules of the sample electrode which enter the ionization chamber in bursts together with the continuous background flow of molecules developed at the walls of the spark chamber by heat or ion bombardment. The molecules traversing the path of the electron beam are ionized and under the influence of the potential gradient established by the repeller and accelerating electrodes, the resultant ions are propelled into the analyzer tube at high velocity. The path of travel of any ion in the analyzer tube is a function of the specific mass of the ion, the magnitude of the accelerating potential and the strength of the transverse magnetic field. By suitable selection of these last two parameters, ions of a given specific mass may be focused on the collector electrode through the resolving electrode.

Discharge of ions at the collector electrode produces an electrical signal which reflects the background ionization as a signal of random frequency including D. C. components and the ionization of the bursts of molecules as an A. C. signal of predetermined frequency. The A. C. components of the collector signal are amplified and by means of' the synchronous detector only the component of the amplified signal corresponding in frequency to the periodicity of the spark is rectified to produce an output signal. In this manner all or substantially all of the spurious effects produced by undirected molecule generation and ionization are excluded from the measurable information developed.

The embodiment of Fig. 1 may be modified by eliminating synchronous detector 38 and providing a tuned amplifier tuned to pass only the A. C. component of the signal received at collector electrode 12 which is of a frequency approximating the frequency of energization of the spark forming electrodes.

A further modification of the apparatus of Fig. l is illustrated in the partial diagram of Fig. 3. The instrument shown in Fig. 3 includes an ion source 50 including a repeller electrode 51 and accelerating electrodes 52, 53 connected to a potential source (not shown) which may be identical to the circuit 22 of Fig. 1. Ion source 50 gives access to an analyzer tube 54 which may be identical to that shown in Fig. 1, including resolving and collector electrodes (not shown). A molecule generating chamber 56 is connected to the ionization chamber giving access through an aperture 50A so that molecules generated in chamber 56 will flow into the ionization chamber and into the influence of an electron beam 57. The electron beam may be formed by means of an electron gun (not shown) of the type illustrated in Fig. 2. A pair of electrodes 58, 59 are mounted in chamber 56 at least one of which again constitutes a sample to be analyzed. As in the previously described embodiment, the ionization chamber, analyzer tube and molecule generating chamber are housed in an evacuable envelope 60. The analyzer tube is also immersed in a transverse magnetic field developed by conventional magnet means (not shown).

The embodiment of Fig. 3 includes an oscillator 62 developing an alternating wave form illustrated at 62A and which is fed into a pulse former 63. The pulse former converts the oscillator signal to a pulsed wave of a configuration illustrated at 63A. A high voltage, high frequency generator 64 develops a high voltage alternating current of the wave form illustrated at 64A which is supplied to the electrodes 58, 59. In this embodiment the high voltage generator is not triggered by the pulse former but develops a continuous high frequency, high voltage wave. The electrodes are vibrated by a coil 66 which is connected to pulse former 63 so as to vibrate electrode 59 at the frequency of the pulse signal 63A. In this manner the same A. C. component of discharge current is developed at the collector electrode of the instrument and is distinguishable by the means illustrated in Fig. l, or by alternative means as described, from the spurious D. C. or random frequency background components.

My invention overcomes a present major defect in the mass analysis of solid samples and increases the aclcuracy and resolution of a solid mass spectrometer to an appreciable degree.

I claim:

1. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a pair of electrodes in the spark chamber, at least one of which is composed of the solid material, means for periodically developing a spark between the pair of electrodes, means for determining the periodicity of the spark, amplifier means connected to the ion collector for amplifying alternating components of ion discharge current, and means for excluding from the amplified signal all frequency components except that fre quency corresponding to the periodicity of the spark.

2. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamher, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a pair of electrodes in the spark chamber, at least one of which is composed of the solid material, means for periodically developing a spark between the pair of electrodes, means for determining the periodicity of the spark, a tuned amplifier connected to the ion collector for amplifying only those alternating components of ion discharge current corresponding in frequency to the periodicity of the spark.

3. In a mass spectrometer for analysis of solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a pair of electrodes in the spark chamber, at least one of which is composed of the solid material, means for periodically developing a spark between a pair of electrodes, means for determining the periodicity of the spark, amplifier means connected to the ion collector for amplifying alternating components of ion discharge cur-- rent, a synchronous detector connected to the output of the amplifier to rectify that part of the amplifier output. signal which is of a frequency corresponding to the periodicity of the spark.

4. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a pair of electrodes in the spark chamber, at least one of which is composed of the solid material, means for periodically developing a spark between the pair of electrodes, oscillator means for establishing the periodicity of the spark, amplifier means conected to the ion collector for amplifying alternating components of ion dis- :harge current, and means interconnected to the oscillator for excluding from the amplified signal all frequency components except that frequency corresponding to the periodicity of the spark.

5. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a

pair of electrodes in the spark chamber, at least one of which is composed of the solid material, a high frequency, high voltage generator connected to the pair of electrodes for developing a spark therebetween, an oscillator connected to interrupt the spark at a predetermined frequency, amplifier means connected to the ion collector for amplifying alternating components of ion discharge current, and means interconnected to the oscillator for excluding from the amplified signal all frequency components except that frequency corresponding to the periodicity of the spark.

6. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a pair of electrodes in the spark chamber, at least one of which is composed of the solid material, a high frequency, high voltage generator connected to means for periodically developing a spark between the pair of electrodes, an oscillator, a pulse former connected between the oscillator and the generator to trigger the generator, amplifier means connected to the ion collector for amplifying alternating components of ion discharge current, and means for excluding from the amplified signal all frequency components except that frequency corresponding to the frequency of oscillation of the oscillator.

7. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamber, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a pair of electrodes in the spark chamber, at least one of which is composed of the solid material, a driving coil mounted to drive one of the pair of electrodes, a high frequency, high voltage generator connected to the electrodes, an oscillator, a pulse former connected between the oscillator and the coil to energize the coil at a predetermined repetition rate, amplifier means connected to the ion collector for amplifying alternating components of ion discharge current, and means for excluding from the amplified signal all frequency components except that frequency corresponding to the frequency of oscillation of the oscillator.

8. In a mass spectrometer for analysis of a solid material, the combination comprising an ionization chamber, means for ionizing molecules in the ionization chamher, an analyzer chamber, an ion collector disposed in the analyzer chamber, means for propelling ions from the ionization chamber into and through the analyzer chamber whereby ions of a given mass-to-charge ratio may be caused to strike and discharge on the ion collector, a spark chamber opening into the ionization chamber, a plurality of electrodes in the spark chamber, at least one of which is composed of the solid material, means for periodically developing a spark between said one electrode and at least one other of said plurality of electrodes, means for determining the periodicity of the spark, amplifier means connected to the ion collector for amplifying alternating components of ion discharge current, and means for excluding from the amplified signal all frequency components except that frequency corresponding to the periodicity of the spark.

No references cited. 

